This invention is concerned with a multi-beam optoelectronic barrier (or light curtain) using LEDs and photodiodes.
As is known, the optoelectronic barrier is based on the principle of maintaining an infrared light ray or beam across a passage which is to be monitored, and of detecting breaks in the ray in order to deliver a signal that the barrier has been infringed.
Due to their immaterial nature, optoelectronic barriers have the important advantage that they do not put any physical obstacles in the passages which are to be protected or monitored, of reacting at high speed and, due to the use of infrared light, of being invisible. Consequently, they are meeting with increasing favour in different applications, such as: accident prevention equipment for access to tool machines, dangerous environments, etc.; theft prevention equipment or equipment for controlling access to restricted areas; access control for highway toll gates; etc.
An optoelectronic barrier comprises several light-emitting diodes (LEDs) generating rays, usually parallel, and directed to respective photodiodes, for monitoring wide passages with a fine raster, i.e. with detection of breaks even when narrowly localized. It is therefore necessary that each photodiode only reads the ray of its associated LED, and to such end the LEDs are energized in a discrete succession, while the corresponding photodiode is enabled in synchronism. Since the photodiodes may be several tens or hundreds, it is known to convey their output signals to multiplexer circuits controlled by an addressing unit, in order to lead the signal from the instantaneously enabled photodiode to a common evaluating circuit, which analyzes the intensity and/or duration of the received signal.
In consideration of the great diversity of applications Listed above, and of the different number of infrared rays (i.e. of LEDs and photodiodes) installed in different situations, the multiplex and control logic must be designed for each case: this circumstance forces the manufacturer to maintain in production a broad range of versions. Moreover, an optoelectronic barrier, once installed, cannot be mollified, i.e. it cannot be adapted to possible physical changes in the protected area: if, say, the access passage to a tool machine is broadened or reduced, the barrier must be replaced with another version, designed for the new size.
The above-described situation is a source of waste of efforts and time devoted to setting-up, and therefore increases the cost of the barrier, particularly in constantly changing environments such as often are industrial plants.